1. Field of the Invention
This invention relates to the field of photonics, and in particular to a method of making specular infrared mirrors for use in optical devices, such as multiplexers and demultiplexers for use in wavelength division multiplex communication systems.
2. Description of Related Art
The manufacture of optical devices such as multiplexers and demultiplexers requires the fabrication of a highly reflective infrared mirror in the 1.55 xcexcm and/or 1.30 xcexcm optical bands. Such a highly reflective infrared mirror is typically required on the sidewall of deep vertical-etched optical components to reflect an infrared laser beam with maximum efficiency.
Typical fabrication techniques of infrared mirrors result in relatively poor surface quality, in lower reflectivity values at 1.55 xcexcm wavelength and in significant optical losses from infrared light scattering from the surface defects.
Optical multiplexers and demultiplexers have been described in the scientific literature for at least 20 years. The following USA patents and published technical information will review the various manufacturing techniques used to produce the highly reflective infrared mirror of multiplexers, demultiplexers and other infrared optical devices:
U.S. Pat. No. 4,274,706, Hughes Aircraft Company
This patent describes the mirror of infrared multiplexers and demultiplexers shown in FIGS. 1a and 1b. The multiplexers and demultiplexers incorporating the mirror allowing the reflection of infrared light are manufactured using a sodium glass microscope slide substrate; a planar wave guide produced by increasing the refractive index of the surface of this substrate to a depth of about 100 xcexcm using an ion exchange process replacing the sodium atoms of the substrate by lithium atoms from a LiSO4 salt heated at about 580xc2x0 C. in oxygen; a grind-and-polished cylindrical shaped surface transverse to the surface of the glass substrate as to achieve the curved wave guide of radius R of FIG. 1a (This cylindrical transverse surface is used to focus the light emanating from the input plane (identified as 14 in FIG. 1b) back to the input plane and has a series of parallel grooves forming a series of pairs of facets of uniform spacing); a replica grating comprising a 0.005 inch thick acetate plastic film having 512 grooves/mm coated with an aluminum as to achieve high reflectivity; and an epoxy glue to bond this transversal cylindrical shaped surface to the replica grating.
This manufacturing technique involves the gluing of an aluminum coated thin flexible material such as an acetate plastic film onto a grind-and-polished sodium glass microscope slide. No further detail on the mirror characteristics and/or fabrication technique is given.
U.S. Pat. No. 4,786,133, Commissariat à l""xc3xa9nergie atomique
This U.S. patent describes the mirror of the infrared multiplexers and demultiplexers shown on FIGS. 2a and 2b. The multiplexers and demultiplexers incorporating the mirror allowing the reflection of infrared light is manufactured using a silicon substrate, identified as 20 in FIG. 2b; a stack of three transparent silica layers, identified as 22, 24 and 26 in FIG. 2b, with the intermediate 4 to 5 xcexcm thick phosphorus-doped silica layer 24 having a 10-3 to 10-2 higher refraction index than the lower 6 to 8 xcexcm thick undoped silica layer 22 and the upper 6 to 8 xcexcm thick undoped silica layer 26 surrounding it; a plurality of optical microguides, identified as G1 to GN in FIG. 2a; a concave and elliptic shaped reflective diffraction grating, identified as R in FIG. 2a, constituted by etched facets etched in the stack of three layers; and an aluminum metal layer, identified as 28 in FIG. 2b. 
This manufacturing technique involves the vertical etching of facets through a three-layer optical waveguide followed by an aluminum coating. No detail is given on the aluminum coating fabrication technique.
U.S. Pat. Nos. 5,450,510, 5,608,826 and 5,793,912, APA Optics, Inc.
These three USA patents describe the mirror of similar variations of the infrared wavelength division multiplexed optical modulator shown in FIGS. 3a and 3b. The infrared wavelength division multiplexed optical modulator incorporating the mirror allowing the reflection of infrared light is assembled using a wavelength dispersive multiplexer transmitter, identified as 21 in FIG. 3a, and consisting of a laser power and laser temperature control circuitry, identified as 23 in FIG. 3a, used to maintain the laser power and temperature at stable pre-set values; a directional coupler controller, identified as 24 in FIG. 3a, used to control the integrated modulator; a semiconductor laser diode, identified as 26 in FIG. 3, maintained at constant temperature as to minimise wavelength variations of about 0.0005 xcexcm/xc2x0 C.; a first reflective holographic diffraction grating, identified as 27 in FIG. 3a, used to demultiplex the various wavelengths from each other with a 0.0007 xcexcm separation by using a series of 6190/cm parallel grooves replicated in its surface and overcoated with a reflecting material such as aluminum; a first collimating optics, identified as 28 in FIG. 3a, used to collimate the output beam of the laser diode; a first focusing optics, identified as 29 in FIG. 3a, used to inject the collimated output beam into the external integrated modulator; a mirror, identified as 30 in FIG. 3a, used to reflect the laser diode beam toward the diffraction grating; an integrated modulator, identified as 31 in FIG. 3b, used to attenuate the various wavelengths of the separated beam and used as a directional coupler of the separated beams into the optical fibre; an optical fibre, identified as 21 in FIG. 3a, used to connect the wavelength dispersive multiplexer transmitter and the wavelength dispersive multiplexer receiver; a wavelength dispersive multiplexer receiver, identified as 22 in FIG. 3a, and consisting of a detector array controller, identified as 25 in FIG. 3a, used to control the detector array; a second reflective holographic diffraction grating, identified as 27xe2x80x2 in FIG. 3a, used to multiplex the various wavelengths together and also consisting in a series of 6190/cm parallel grooves ruled or replicated in its surface and overcoated with a reflecting material such as aluminum; a second collimating optics, identified as 28xe2x80x2 in FIG. 3a, used to focus the dispersed wavelengths onto the detector array; a second focusing optics, identified as 29xe2x80x2 in FIG. 3a, used to collimate the multiple wavelength light coming out of the optical fibre with a minimum angular dispersion; a detector array, identified as 32 in FIG. 3a, used to detect the dispersed longitudinal modes; and the two reflective holographic diffraction gratings of this infrared wavelength division multiplexed optical modulator involve an aluminum coating. No detail is given on the aluminum coating fabrication technique.
The highly reflective infrared mirror from Newport Corporation
Newport Corporation, Irvine, Calif., is a worldwide manufacturer and distributor of precision components and systems used for development and application of laser and optical technologies in semiconductor manufacturing and testing, fiber optic communications and other commercial applications. The reflectivity spectra of the ER.1 enhanced aluminum coating near infrared mirror is shown in FIG. 4.
These reflectivity spectra will be used as comparative reference the results for the present invention.
Marxer C. and Al, Vertical mirrors fabricated by deep reactive ion etching for fiber-optics switching applications. Journal of Microelectromechanical Systems, Vol 6 (3), pp. 277-285. September 1997
This paper describes the characteristics and performance of various metal-coated silicon mirrors to be used for electrostatic switches capable of switching 1.3 xcexcm infrared light from optical fibres. The electrostatic switch is shown in FIG. 5a. This paper also describes different metal coatings such as gold, aluminum, nickel and chromium and concludes that as shown in FIG. 5b, the 1.3 xcexcm reflectivity of aluminum-coated silicon increases with increasing aluminum thickness but saturates to the reflectivity of bulk aluminum when the thickness attains 40 nm; only 100 nm of aluminum is required as to prevent the transmission of less than 1 ppm (60 dB isolation) at 1.3 xcexcm wavelength; the other two metals, nickel and chromium, exhibit inferior reflectivity values at 1.3 xcexcm wavelength even for much thicker mirrors; higher reflectivity values are associated to thicker aluminum but the surface roughness associated with thicker aluminum induces a non-specular reflectivity and overall light loss due to diffused light scattering estimated by the following formula:
Pscat=Ptot{1xe2x88x92exp[xe2x88x92(4xcfx80"sgr"cosxcex81/xcex)2]}
where Pscat is the flux of light scattered away from the specular direction, Ptot is the total reflected flux, "sgr" is the RMS surface roughness of the mirror, xcex81 is the incident angle and xcex is the wavelength of light.
The surface roughness can be measured using an Atomic Force Microscope (AFM) profiling of the aluminum surface and the reduction of the surface roughness is key to achieve a highly reflective specular infrared mirror. An example of such AFM profiling is shown in FIG. 5c. 
An object of the present invention is to provide an improved fabrication process for a low surface roughness highly-reflective specular infrared mirror so as to allow the fabrication of optical devices such as multiplexers, demultiplexers and other optical devices operating in the 1.55 xcexcm and/or 1.30 xcexcm optical bands with minimum optical losses.
The present invention provides a novel approach for producing an atomic scale surface roughness aluminum mirror with high infrared specular reflectivity, for example, using a commercially available M2i cluster tool manufactured by Novellus Systems in California, USA. This highly reflective infrared mirror is to be coated onto the facets of a deep-etched grating such as the one shown in FIG. 6.
Accordingly the present invention provides a method of making highly reflective mirrors on a wafer in the manufacture of photonic devices, comprising the steps of preheating a wafer to remove adsorbed volatile contaminants at a temperature between about 300 and 600xc2x0 C.; etching the wafer surface at a temperature between about 300 and 600xc2x0 C. to remove absorbed and chemically absorbed contaminants in the presence of a glow-discharge to reduce poisoning; thoroughly cooling the wafer surface so as to as reduce the surface mobility of the impinging metal atoms during a subsequent metallic deposition; carrying out a deposition on the cooled wafer of a gettering layer for gettering at least one contaminant selected from the group consisting of hydrogen, oxygen and nitrogen; depositing a metallic reflective layer in a deposition chamber; and removing the wafer from the deposition chamber to prevent excessive bulk oxidation.
The invention is useful in the manufacture of echelle gratings in multplexers and demultplexers, especially in the infrared region.